This project aims to test our hypothesis that S1, a sister gene of EF- 1alpha, functions as a candidate "maintenance gene" to maintain the long-lived and terminally differentiated state for myotubes in mature muscle. During development in muscle, S1 expression is activated with a concomitant decline in EF-1alpha protein, but not message, level, suggesting a posttranscriptional suppression of the regulation of EF- 1alpha abundance, and resulting a shift in EF-1alpha ratio from high to low. The low EF-1alpha/S1 ratio persists in adult muscle except in the cases of marcaine-induced injury, long-term denervation, or extreme old age. We also found that stable transfection of S1 into S1- negative mouse fibroblasts induces suppression of endogenous EF- 1alpha. Experiments planned are to test our working hypothesis that in myotubes, S1 may function as a "dimmer switch", dampening EF- 1alpha activity by reducing its protein abundance via translational suppression and/or rapid protein turnover. This dampening action may leave EF-1alpha restrained in not only its traditional function in protein translation, but also other known functions such as involvement in signal transduction, severing microtubles, or bundling actin filaments, all of which may result in establishing an even gear range favorable to the long-term nonapoptotic state in mature muscle. The specific aims formulated are: 1. To test whether posttranscriptional suppression of EF-1alpha protein level in adult muscle is developmentally-and S1 gene expression-dependent; 2. To determine the regulatory mode for the posttranscriptional suppression of EF- 1alpha protein level; 3. To investigate how S1 gene expression is related to this suppression of EF-1alpha protein abundance, and to identify responsible putative cis-element(s) and trans-factor(s); 4. Testing, in cultures, the hypothesis that muscle fibers' atrophy is due to myotube apoptosis and replicative senescence of the satellite cell population; 5. to test the hypothesis that a change from low to high EF-1alpha/S1 ratio is involved in the activation of death in cultured myotubes; and 6. To investigate the regulatory mechanism(s) governing the shift of EF-1alpha/S1 ratio from low to high during injury, long-term denervation, and aging. Studying the mechanism(s) regulating the EF-1alpha/S1 ratio will ultimately allow the design of molecular manipulation to control or slow down muscle degeneration, a condition plaguing most of the elderly.